Distributed MIMO based on access point collaboration

ABSTRACT

A method for communication in a WLAN includes associating a client station (STA) with a basic service set (BSS) of a first access point (AP), having first antennas, in a wireless local area network (WLAN). A second AP, having second antennas, in the WLAN is synchronized with the first AP. Distributed beamforming parameters are computed over a group of the first antennas and the second antennas. Data for transmission to the STA are distributed to both the first AP and the second AP. The distributed data are distributed to the STA from the first AP via the first antennas in the group and the second AP via the second antennas in the group in synchronization in accordance with the distributed beamforming parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/677,949, filed May 30, 2018; U.S. Provisional PatentApplication 62/694,800, filed Jul. 6, 2018; U.S. Provisional PatentApplication 62/730,407, filed Sep. 12, 2018; U.S. Provisional PatentApplication 62/774,782, filed Dec. 3, 2018; and U.S. Provisional PatentApplication 62/783,144, filed Dec. 20, 2018. All of these relatedapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to communication networks, andparticularly to apparatus and methods for spatial multiplexing ofcommunications between access points and client stations.

BACKGROUND

Multiple-Input Multiple-Output (MIMO) is an important component for manymodern wireless technologies. MIMO increases the data-carrying capacityof a transmitter-receiver link by using multiple antennas to transmitand receive signals over multiple different paths. In wireless localarea networks (WLANs, also known as Wi-Fi), for example, the IEEE802.11n standard defines MIMO communications between an access point(AP) and a client station (STA) with multiple antennas, which enablecompatible products to achieve higher data rates than earlier-generationWi-Fi equipment.

Multi-user MIMO (MU-MIMO) is a multiple-access enhancement of MIMOtechnology that uses spatial multiplexing over multiple antennas toenable multiple transmitters to transmit signals simultaneously to thesame receiver and/or multiple receivers to receive signalssimultaneously from the same transmitter. MU-MIMO for WLANs is defined,for example, in the IEEE 802.11ac and 802.11ax specifications, whichpermit multiple client stations (STAs), to transmit or receiveindependent data streams simultaneously to or from the same AP byspace-division multiple access (SDMA).

Another type of MIMO, which has been developed for use in Long TermEvolution (LTE) cellular networks, is cooperative MIMO, in whichmultiple devices, each with its own antenna or antennas, are groupedinto a virtual antenna array to achieve MIMO communications. Thesecooperative MIMO functionalities take advantage of the fact that allbase station clocks in an LTE network are precisely synchronizedcontinuously. For example, LTE 4G standards have introduced the use ofCoordinated Multipoint (CoMP) communications, in which neighboringcellular base stations share data and channel state information in orderto coordinate their downlink transmissions to user equipment and tojointly process the uplink signals that they receive. CoMP and othercooperative MIMO schemes can increase the data capacity of wirelessnetworks significantly by enhancing exploitation of the spatial domain,but at the expense of increased complexity in control and communicationbetween the cooperating transmitters and receivers.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved apparatus and methods for wireless networkcommunications.

There is therefore provided, in accordance with an embodiment of theinvention, a method for communication in a WLAN, which includesassociating a client station (STA) with a basic service set (BSS) of afirst access point (AP), having first antennas, in a wireless local areanetwork (WLAN). A second AP, having second antennas, in the WLAN issynchronized with the first AP. Distributed beamforming parameters arecomputed over a group of the first antennas and the second antennas.Data for transmission to the STA are distributed to both the first APand the second AP. The distributed data are transmitted to the STA fromthe first AP via the first antennas in the group and the second AP viathe second antennas in the group in synchronization in accordance withthe distributed beamforming parameters.

In some embodiments, synchronizing the second AP includes exchangingsynchronization signals between the first AP and the second AP over abackbone network. In one embodiment, exchanging the synchronizationsignals includes synchronizing respective clocks and frequency offsetsof the second AP with the first AP.

In a disclosed embodiment, associating the STA includes providing fromthe first AP to the STA a BSS identifier (BSSID) of the first AP, andtransmitting the distributed data includes transmitting signals fromboth the first AP and the second AP using the BSSID of the first AP.Typically, transmitting the signals includes conveying encodingparameters to the second AP that are identical to the encodingparameters applied by the first AP, and encoding the distributed datafor transmission by both the first AP and the second AP using theidentical encoding parameters.

In some embodiments, the first and second APs have respective first andsecond numbers of antennas, and the group includes a third number of theantennas, which is greater than the first and second numbers.

In one embodiment, computing the distributed beamforming parametersincludes receiving from the STA, at the first AP, a request to establishthe group of the antennas together with the second AP, and setting upthe group in response to the request. Alternatively, computing thedistributed beamforming parameters includes collecting information atthe first AP with respect to neighboring APs in the WLAN, and selectingthe second AP responsively to the collected information.

In a disclosed embodiment, computing the distributed beamformingparameters includes transmitting null data packets (NDPs) from both thefirst AP and the second AP to the STA, receiving channel feedback fromthe STA in response to the NDPs, and computing, responsively to thechannel feedback, a steering matrix to be applied to downlink signalvectors.

Alternatively, computing the distributed beamforming parameters includesreceiving uplink signals from the STA at the first AP and the second AP,and computing, by implicit beamforming based on the uplink signalsreceived at both the first AP and the second AP, a steering matrix to beapplied to downlink signal vectors. In a disclosed embodiment, computingthe steering matrix by implicit beamforming includes computingcalibration factors among the antennas of each of the first and secondAPs individually, and then computing a common calibration factor acrossboth of the first and second APs.

Typically, transmitting the distributed data includes encoding the datafor transmission at both the first AP and the second AP identically inaccordance with a specified procedure and encoding parameters.Additionally or alternatively, distributing the data for transmissionincludes broadcasting complex modulated symbols, which encode the data,to be transmitted by the first AP and the second AP.

In one embodiment, transmitting the distributed data includes applying,by at least one of the APs, a pre-compensation for a frequency offset insignals that are transmitted from the APs to the STA.

Additionally or alternatively, computing the distributed beamformingparameters includes determining amplitude and phase values to be appliedin transmitting the data via each of the first and second antennas.Further additionally or alternatively, computing the distributedbeamforming parameters includes computing precoding matrices to beapplied to the data for transmission.

In some embodiments, associating the STA includes associating at least afirst STA and a second STA with the BSS of the first AP, and computingthe distributed beamforming parameters includes computing different,respective first and second beamforming parameters for downlinkcommunications with the first STA and the second STA. In one embodiment,the method further includes synchronizing a third AP, having thirdantennas, in the WLAN with the first AP, wherein computing thedifferent, respective first and second beamforming parameters includescomputing the first beamforming parameters over a first group of thefirst antennas and the second antennas, and computing the secondbeamforming parameters over a second group of the first antennas and thethird antennas, and wherein transmitting the distributed data includestransmitting first signals to the first STA from the first and secondAPs in accordance with the first beamforming parameters and transmittingsecond signals to the second STA from the first and third APs inaccordance with the second beamforming parameters.

There is also provided, in accordance with an embodiment of theinvention, a method for communication, which includes respectivelyassociating each of a plurality of client stations (STAs), including atleast a first STA and a second STA, with at least one basic service set(BSS) of at least one access point (AP) among a plurality of APs, havingrespective antennas, in a wireless local area network (WLAN). The APs inthe WLAN are synchronized prior to transmitting data to the STAs.Respective first and second groups of the antennas are defined fordownlink communication with the first and second STAs, each of thegroups including antennas belonging to at least two of the APs. Firstdistributed beamforming parameters are computed over the first group ofthe antennas for downlink communications with the first STA, and seconddistributed beamforming parameters, different from the first distributedbeamforming parameters, are computed over the second group of theantennas for downlink communications with the second STA. Respectivedata for transmission to the first and second STAs are distributed tothe APs to which the antennas in the respective first and second groupsbelong. The distributed data are transmitted to the first and secondSTAs from the APs via the first and second groups of the antennas insynchronization in accordance with the first and second distributedbeamforming parameters.

In a disclosed embodiment, transmitting the distributed data includestransmitting signals simultaneously to both of the first and second STAsvia the first and second groups of the antennas.

In one embodiment, respectively associating each of the plurality of theSTAs includes associating the first STA and the second STA with the sameBSS. Alternatively, the plurality of APs includes at least a first APand a second AP, and respectively associating each of the plurality ofthe STAs includes associating the first STA with a first BSS of a firstAP and associating the second STA with a second BSS, different from thefirst BSS, of the second AP.

There is additionally provided, in accordance with an embodiment of theinvention, a wireless local area network (WLAN) system forcommunication, including a first access point (AP), having firstantennas and a basic service set (BSS), which is configured to associatewith a client station (STA) in a wireless local area network (WLAN). Asecond AP in the WLAN, having second antennas, is configured tosynchronize with the first AP. At least one processor is configured tocompute distributed beamforming parameters over a group of the firstantennas and the second antennas, and to distribute data fortransmission to the STA to both the first AP and the second AP, whereinthe first AP and the second AP transmit the distributed data to the STAvia the first antennas in the group and the second antennas in thegroup, respectively, in synchronization in accordance with thedistributed beamforming parameters.

In one embodiment, the at least one processor is disposed in at leastone of the first and second APs. Alternatively or additionally, the atleast one processor is disposed in a hardware unit that is separate fromthe first and second APs.

There is further provided, in accordance with an embodiment of theinvention, a wireless local area network (WLAN) system, including aplurality of access points (APs) having respective antennas, theantennas being configured to associate with a plurality of clientstations (STAs) in a wireless local area network (WLAN), the WLANincluding at least a first STA and a second STA, such that each STA isassociated with at least one basic service set (BSS) of at least one APamong the plurality. At least one processor is configured to synchronizethe APs in the WLAN, and to define respective first and second groups ofthe antennas for downlink communication with the first and second STAs,each of the groups including antennas belonging to at least two of theAPs, and to compute first distributed beamforming parameters over thefirst group of the antennas for downlink communications with the firstSTA, and second distributed beamforming parameters, different from thefirst distributed beamforming parameters, over the second group of theantennas for downlink communications with the second STA, and todistribute respective data for transmission to the first and second STAsto the APs to which the antennas in the respective first and secondgroups belong. The access points transmit the distributed data to thefirst and second STAs via the first and second groups of the antennas insynchronization in accordance with the first and second distributedbeamforming parameters.

In one embodiment, the distributed beamforming parameters include asteering matrix to be applied to downlink signal vectors transmitted tothe first and second STAs.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic pictorial illustration of a wireless network system,in accordance with an embodiment of the invention;

FIG. 2 is a block diagram showing functional components of the system ofFIG. 1, in accordance with an embodiment of the invention;

FIG. 3 is a flow chart that schematically illustrates a method fordistributed MIMO (DMIMO) communications that can be implemented, forexample, in the system of FIG. 1, in accordance with an embodiment ofthe invention;

FIGS. 4A and 4B are a ladder diagram showing messages exchanged in aprocess of DMIMO communications, which can be used in implementing themethod of FIG. 3, in accordance with an embodiment of the invention;

FIGS. 5 and 6 are ladder diagrams showing messages exchanged in aprocess of synchronization between access points, which can be used inimplementing the method of FIG. 3, in accordance with embodiments of theinvention;

and

FIG. 7 is a flow chart that schematically illustrates a method fordownlink multi-user MIMO (DL-MUMIMO) communications that can beimplemented, for example, in the system of FIG. 1, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

MU-MIMO technologies, as specified in the 802.11ac and 802.11axstandards, for example, increase the speed and capacity of Wi-Finetworks. Existing approaches, however, still fail to take fulladvantage of available spatial resources, and therefore cannot fullyaddress the growing bandwidth needs in crowded wireless networkenvironments, such as large offices and public spaces. In these existingapproaches, each AP in the WLAN operates independently of its neighbors,with its own basic service set (BSS) and its own frequency allocation.Each STA can associate and communicate with only one AP at any giventime, and signals that the STA receives from other APs are treated asinterference.

Embodiments of the present invention that are described herein addressthis problem by enabling APs in a WLAN to collaborate in serving STAs,by means of distributed MIMO (DMIMO) schemes among APs whose normaloperation is not mutually synchronous. In DMIMO, two or more APstransmit data to a STA in synchronization, so that the antennas of thecollaborating APs operate together as a virtual antenna array. Byextending the antenna arrays in the WLAN in this manner, the DMIMOapproach is able to take advantage of unused AP transmitter capacity toincrease beamforming gain and thus increase downlink data throughput,reliability and range of the network. The STAs receive signals withhigher power and improved signal/interference ratio. For example, usingtwo APs together, each with four antennas, in DMIMO transmission to aSTA operating in accordance with the 802.11ax standard, the inventorswere able to achieve a peak transmission throughput of 2000 Mbps at arange of 50 feet, as opposed to only 1400 Mbps with a single AP. In thissame configuration, the maximum range for DMIMO transmission at 250 Mbpswas 210 feet, as opposed to only 150 feet using a single AP.

To realize these benefits, it is necessary that the APs in a given WLANenvironment operate in mutual synchronization in contrast to theindependent, unsynchronized operation of APs in WLANs that are known inthe art. To achieve this synchronization, the APs communicate amongthemselves over a backbone network, such as a wired network (forexample, an Ethernet LAN, to which APs are connected for purposes ofInternet access in most deployments), or a wireless network, which maybe a dedicated network or implemented over a channel of the WLAN itself.The collaborating access points communicate over the backbone network toestablish DMIMO groups, to synchronize time, frequency and carrier phaseacross the groups, and to distribute data for transmission to the clientSTAs, all in real time. Beamforming parameters for each DMIMO client STAare established over the corresponding group of APs and adjusted asnecessary to adapt to channel fluctuations.

In some embodiments, all of these functions are carried out among theAPs without requiring any cooperation by client STAs, beyond thesignaling capabilities that are already mandated by existing IEEE 802.11standards. In other words, the clients need not request DMIMO service oreven be aware that they are being served by more than a single AP. Thechanges required in an existing WLAN to offer DMIMO service can beimplemented entirely in software or firmware, taking advantage of theexisting infrastructure, in accordance with an embodiment.

In the disclosed embodiments, DMIMO service is initiated when a givenclient STA associates with a basic service set (BSS) of a given AP in aWLAN. One or more other, neighboring APs are synchronized and pairedwith the given AP, typically by synchronization and pairing processesthat take place periodically within the WLAN. (Alternatively oradditionally, these processes could take place ad hoc, in response to arequest by the STA, for example, although such a request may not berequired in at least some embodiments). An AP controller decides whetherthe given STA is to receive DMIMO service and, if so, selects theneighboring AP (or possibly multiple neighboring APs) that is toparticipate in providing the DMIMO service. The AP controller may be asoftware or firmware function implemented on the host processors of theAPs themselves, or it may alternatively be implemented in a separatehardware unit.

After selecting the APs that are to participate in the DMIMO service,the controller and/or APs compute distributed beamforming parametersover the group of antennas belonging to the APs. (This antenna group cantypically comprise a larger number antennas than any single AP hasitself, thus improving the beamforming performance still further.) Thecontroller may distribute data for transmission to the STA to all of theAPs in the groups. Alternatively, the data may originate from anothersource (such as an internet server), and the controller decides theschedule of packet transmission by each AP. These APs transmit the datato the STA via their respective antennas in synchronization using thedistributed beamforming parameters. The transmissions from all the APsuse the BSS identifier (BSSID) of the given AP, with which the STA hasassociated, so that the STA can receive and process the downlink signalsin the standard fashion, without any awareness that they weretransmitted by multiple APs.

Although the embodiments described below relate, for the sake ofsimplicity and clarity, mainly to the steps involved in DMIMOtransmission to a single STA, in practice the APs in the WLAN willprovide DMIMO service to multiple STAs concurrently. For example, whenmultiple STAs associate with the BSS of a given AP and are served by thegiven AP together with a neighboring AP, different, respectivebeamforming parameters will be computed and implemented for downlinkcommunications with each of the STAs. In some cases, differentneighboring APs will participate in the respective DMIMO groups servingdifferent STAs.

Scenarios of this sort, in which a groups of APs transmits DMIMO signalssimultaneously to multiple STAs, are referred to herein as distributeddownlink multi-user MIMO (DL-MUMIMO). In some of these scenarios, theparticipating STAs may be associated with the (different) BSSIDs ofdifferent APs. In this case, for example, a first STA may associate witha first AP and may receive DMIMO transmissions from a second STAtogether with the first STA, as described above, using the BSSID of thefirst AP. At the same time, a second STA may associate with the secondAP, and receive DMIMO transmissions from the first and second APs usingthe BSSID of the second AP. More complex scenarios, involving multipleother APs in the WLAN, can be supported in the same manner, with arespective antenna group assigned and beamforming parameters computedfor each client STA. In this manner, the entire, collective downlinkcapacity of the APs in the WLAN may be better used to exploit availablespatial, temporal and frequency resources; in some embodiments all suchspatial, temporal and frequency resources are used.

Reference is now made to FIGS. 1 and 2, which schematically illustrate awireless network system 20, in accordance with an embodiment of theinvention. FIG. 1 shows an overview of the operating environment, whileFIG. 2 is a block diagram showing functional components of the system.

In system 20, APs 22, 24, 26, . . . , having respective arrays ofantennas 28, collaborate in providing DMIMO service to client STAs 30,32, 34, . . . . Each STA typically associates with the AP from which itreceives the strongest signals, but may then receive, in addition, DMIMOsignals from one or more of the antennas 28 of one or more additionalAPs in its vicinity. In the example shown in FIG. 1, STA 30 (labeledSTA1 for convenience) receives signals from both AP 22 (AP1) and AP 24(AP2). Meanwhile, STA 34 (STA2) receives signals from AP 24 and AP 26(AP3). As noted earlier, APs 22, 24, 26, . . . , are connected by abackbone network 36, over which they transmit and receivesynchronization and control signals, as well as data. APs exchangesynchronization communications over backbone network 36 prior tocommencing DMIMO communication with a STA.

As shown in FIG. 2, STA 30 comprises a network interface (NI) 40, whichcomprises PHY and medium access control (MAC) interfaces 42 and 44, inaccordance with the IEEE 802.11 specifications. PHY interface 42comprises one or more radio transceivers 46, which are connected toantennas 48. In the pictured embodiment, PHY interface 42 comprises twosuch transceivers, each with its own antenna. Alternatively, larger orsmaller numbers of transceivers and antennas may be used, with one ormore antennas connected to each transceiver. The internal constructionof STAs 32 and 34 is typically similar to that of STA 30. In general,the components of PHY and MAC interfaces 42 and 44 are implemented indedicated or programmable hardware logic circuits, on a singleintegrated circuit chip or a set of two or more chips.

A host processor 50 passes data to network interface 40 for transmissionover the air to target AP receivers, and receives incoming data fromnetwork interface 40. Host processor 50 typically comprises amicroprocessor, along with a suitable memory and other resources (notshown), and is programmed in software or firmware to carry out variouscontrol and communication functions in STA 30. The software may bestored in tangible, non-transitory computer-readable media, such as asuitable RAM or ROM memory. Host processor 50 may be implementedtogether with the elements of network interface 40 in a singlesystem-on-chip (SoC), or as a separate chip or chip set.

The internal construction of APs 22, 24, 26, . . . , as shown in theinset in FIG. 2, is generally similar to that of STA 30. AP 22 comprisesa network interface (NI) 52, which comprises PHY and MAC interfaces 54and 46, again in accordance with the applicable IEEE 802.11specifications. PHY interface 54 comprises multiple radio transceivers58, which are connected to antennas 28. In the pictured embodiment, PHYinterface 42 comprises four such transceivers, each with its ownantenna. Alternatively, larger or smaller numbers of transceivers andantennas may be used, with one or more antennas connected to eachtransceiver. In general, the components of PHY and MAC interfaces 54 and56 are implemented in dedicated or programmable hardware logic circuits,on a single integrated circuit chip or a set of two or more chips.

A host processor 60 passes data to network interface 52 for transmissionover the air to target receivers, and receives incoming data fromnetwork interface 52. Host processor 60 also communicates with other APsover backbone network 36, using a backbone interface 62, such as anEthernet interface, a WLAN interface, or a mesh network interface. Whenbackbone network 36 is a wired Ethernet network, for example, backboneinterface 62 comprises suitable Ethernet PHY and MAC circuits.Alternatively, backbone interface 62 may comprise a wireless networkinterface, and may even be integrated with NI 52 when backbone network36 is integrated into the WLAN. In this case, backbone interface 62 mayuse the bandwidth of the WLAN in particular time slots (i.e., TDMA)and/or within a particular frequency channel or channels (FDMA).

Host processor 60 typically comprises a programmable processor, alongwith a suitable memory and other resources (not shown), and isprogrammed in software or firmware to carry out various control andcommunication functions in AP 22. These functions include theDMIMO-related functions carried out by the AP in connection withsynchronization and computation of beamforming and other parameters,which are then implemented by NI 52. The software run by host processor60 is suitably stored in tangible, non-transitory computer-readablemedia, such as a suitable RAM or ROM memory in various embodiments. Hostprocessor 60 may be implemented together with the elements of networkinterface 52 and backbone interface 62 in a single system-on-chip (SoC),or as a separate chip or chip set.

A DMIMO controller 64, which may be a standalone device or may beintegrated in one or more of the APs, is connected to backbone network36 and coordinates functions such as pairing of APs to serve STAs insystem 20 and distributing data to the APs for transmission to the STAsthey are serving. Controller 64 may comprise a standalone unit,including a processor executing the required software code and asuitable wired or wireless interface to backbone network 36 (similar tointerface 62, as described above). Alternatively or additionally, DMIMOcontroller 64 may be embedded as an additional processor in one or moreof APs 22, 24, 26, . . . , or as a software process running on hostprocessor 60 of one or more of the APs, or as a distributed processrunning on multiple APs or all of the APs. All such implementations areconsidered to be within the scope of the invention, and references to“DMIMO controller 64” should be understood as encompassing all of theseimplementation options, unless the context indicates otherwise.

The DMIMO-enabled APs in system 20 share some basic information with oneanother, such as geographical location, protocols supported, and DMIMOcapabilities and support. Some APs may claim the capability to act asthe controller to form a DMIMO network.

Alternatively, a gateway router (not shown) in system 20 can act ascontroller. Since all data coming from outside system is routed by thisdevice, the gateway router can distribute the DMIMO data and provideinstructions to the DMIMO APs as to how they should cooperate.

FIG. 3 is a flow chart that schematically illustrates a method fordistributed MIMO (DMIMO) communications, in accordance with anembodiment of the invention. The method is described hereinbelow, forthe sake of convenience and clarity, with specific reference to theelements of system 20, as shown in FIGS. 1 and 2. Alternatively, theprinciples of the present method may be implemented in other suitablesorts of WLAN environments.

To enable DMIMO operation, the local clocks of APs 22, 24, 26, . . . ,in system 20 are synchronized with one another, at an AP synchronizationstep 70. This step is typically carried out by exchangingsynchronization signals among the APs over backbone network 36, forexample, by transmitting and receiving synchronization packets using thePrecision Time Protocol (PTP), as is known in the art. Step 70 isperformed at system start-up, and may be repeated periodicallythereafter in order to keep the clocks within appropriate tolerancelimits. Additionally or alternatively, the local clocks of APs 22, 24,26, . . . , in system 20 can be synchronized on demand in response totrigger packets transmitted by client STAs, as described furtherhereinbelow. After receiving the trigger packet, the APs synchronize tothe clock of the client and then synchronously transmit the downlinkpacket.

In order to initiate data communications, any given STA firstestablishes an association with an AP, at an association step 72. Itwill be assumed in this example, without loss of generality, that STA1associates with the BSS of AP1, using the standard 802.11 communicationprotocol. As part of the association process, STA1 receives the BSSID ofAP1 and will use this BSSID in subsequent communications.

Not all communications in system 20 are necessarily subject to DMIMOtransmission. For example, when the STA is located in close proximity toa given AP and receives adequate signals from that AP alone, DMIMO maynot be called for, and communications can continue using theconventional, single-AP model. Conditions and methods for triggeringDMIMO operation under appropriate circumstances—by either AP1 orSTA1—are described further hereinbelow.

When controller 64 receives a DMIMO trigger of this sort, it choosesanother AP to pair with the AP that is already serving the STA, at an APpairing step 74. For example, in the present case, controller 64 maydetermine that AP2 is in proximity to STA1, and may therefore pair AP2with AP1 for the purpose of DMIMO communication with STA1. This pairingstep may likewise be repeated periodically, for example in case STA1moves within the service area of system 20, whereupon a differentpairing is preferred. Additionally or alternatively, controller 64 maymaintain a network database that lists neighboring APs for purposes ofDMIMO pairing. Further additionally or alternatively, the client STA,such as STA1 in the present example, may detect and selecting theneighboring AP or APs to be paired with AP1 for DMIMO service.

The pair of AP1 and AP2 together have a larger number of availableantennas than either of the APs individually, and some or all of theseantennas can be used to serve STA1. For optimal performance (though notnecessarily), the number of antennas 28 and corresponding transceivers58 that will serve STA1 in DMIMO operation will typically be greaterthan the number of antennas and transceivers in either AP individually,thus achieving greater transmitted power and improved directionality.

Host processors 60 in AP1 and AP2 compute beamforming parameters foreach antenna 28 that is going to transmit downlink signals to STA1, at abeamforming computation step 76. The beamforming parameters determinethe amplitudes and relative phases of the signals that are to betransmitted by each of the participating antennas 28, in order to focusthe combined antenna beams precisely at the location of STA1 whilecompensating for channel variations. Host processors 60 may compute thebeamforming parameters, for example, on the basis of uplink signalsreceived from STA1 at the antennas 28 in question, or alternatively bytransmitting test downlink signals to STA1 from each antenna, andreceiving channel feedback in return. The participating APs can computethe beamforming parameters over all the participating antennas, and thusfor a higher spatial dimension than the number of transmit antennas(NumT×Ant) in each AP. This high-dimension computation is possible ifthe STA provides channel sounding feedback of sufficiently high order.Thus, multiple APs can truly beamform as though they were a singledevice with a higher value of NumT×Ant than the individual APs.

Controller 64 distributes data to AP1 and AP2 for downlink transmissionto STA1, at a data distribution step 78. (As noted earlier, thefunctions of controller may be embodied in one or more of the APs or ina separate device; and the data may originate from another source, suchas an Internet gateway.) AP1 and AP2 encode the data in appropriatesignals for transmission to STA1, and apply the respective beamformingparameters to the signals in each of transceivers 58 in AP1 and AP2 thatis participating in the DMIMO group. AP1 and AP2 then transmit thesignals simultaneously, in mutual synchronization, via the correspondingantennas 28, to STA1, at a downlink transmission step 80. STA1consequently receives the data with greater signal power and betterspatial directionality than could have been provided by AP1 alone.

FIGS. 4A and 4B are a ladder diagram showing messages exchanged in aprocess of DMIMO communications, in accordance with an embodiment of theinvention. FIGS. 4A/B give further details of possible implementationsof the process outlined in FIG. 3.

After the client STA (for example, STA1) has associated with the BSS ofthe primary AP (for example, AP1) at step 72 (FIG. 3), DMIMO operationis initiated by a signal referred to as a DMIMO trigger 90, which canoriginate either from the client or from the AP with which the clientSTA previously associated (referred to herein as the associated AP). Inthe former case, referred to as client-initiated DMIMO, STA1 transmits arequest to AP1 to establish a DMIMO group, based on information that ithas collected regarding neighboring APs in the WLAN. In an embodiment,STA1 may initiate DMIMO particularly when it senses that the currentdownlink transmission quality is inadequate, for example when thesignals from AP1 are weak (meaning that transmission range extension maybe needed) or when a higher downlink transmission rate is required. Inan alternative embodiment, in AP-initiated DMIMO, AP1 receivesinformation with respect to its neighboring APs, and the DMIMO group isformed accordingly. AP-initiated DMIMO can be implemented without activeinvolvement by the STA in setting up the DMIMO group, meaning that itcan be used with unmodified, legacy STAs. This sort of operation with alegacy STA is possible because the signaling between the APs and the STAcomplies with existing protocols, and the STA need not be aware that theantennas from which it receives the downlink signals are physicallyattached to different APs.

Following DMIMO trigger 90, actual DMIMO operation begins with an uplinktransmission 92 from STA1 to AP1. Transmission 92 can simply be arequest for data in the case of AP-initiated DMIMO. In this case, AP1can ask STA1 to provide a STA report identifying neighboring APs (byaddress and BSSID, for example) and the received channel power indicator(RCPI) for each AP. Based on this report, AP1 collects and sendsinformation 94 for use in AP selection to controller 64. The controllerthen selects a neighboring AP to join into the DMIMO group, for exampleAP2, based on factors such as the strength of the RCPI and the currentload on the neighboring AP in terms of the number of other STAs that areassociated with it.

Alternatively, in the case of client-initiated DMIMO, transmission 92may comprise a DMIMO trigger frame or a null-data packet (NDP) for thepurpose of DMIMO initiation. The trigger frame may be of the form, forexample, that is specified in section 9.3.1.22 of the IEEE 802.11ax/D3.3draft standard (December 2018), which is incorporated herein byreference. STA1 may send transmission 92 not only to the associated AP1,but also to other APs operating in the same frequency band. STA1 mayobtain a report from AP1 of eligible neighboring APs and may then sendtransmission 92 on this basis, or it may assemble the list ofneighboring APs itself based on transmissions received by STA1. In thismanner, STA1 may actually select the neighboring AP to join into theDMIMO group. AP1 and the other APs receiving transmission 92 from STA1then send information 94 to controller 64.

Once the DMIMO group has been selected—by any of the methods outlinedabove—controller 64 broadcasts signaling information 96 to the selectedAPs, assumed to be AP1 and AP2 in the present example. This broadcasttakes place by transmission of appropriate packets over backbone network36. As noted earlier, the backbone network may be wired or wireless, andthe controller that performs the broadcast can be either embedded in oneor more of the APs or in a separate device. Signaling information 96indicates the medium access control (MAC) parameters to be applied byboth AP1 and AP2 in communicating with STA1. In one embodiment,signaling information 96 may also include synchronization informationfor synchronizing the timing of subsequent downlink transmissions by AP1and AP2.

Both AP1 and AP2 must use the same MAC parameters, including the BSSIDof AP1, so that the downlink signals appear to STA1 as though they hadall originated from AP1. Either AP1 or controller 64 provides the BSSIDand other encoding parameters to the second AP, such as the MCS, NSS,FEC parameters, MAC header, and encryption key. Both APs will encode thedownlink packets to STA1 using the identical encoding parameters, alongwith same BSSID. AP1 and AP2 will then retrieve and transmit the samedata to STA1, with the same padding, encoding, interleaving andencryption. The “capabilities” field in the downlink packets willreflect the number of antennas (Ntx) and the number of data streams(Nss) transmitted over all of antennas 28 from both APs in the DMIMOgroup. (The Nss feature of the capability field is defined in section27.15.4.2 of the above-mentioned IEEE 802.11ax standard, which islikewise incorporated herein by reference.) AP1 and AP2 will alsocompute and apply carrier frequency offset (CFO) and sampling frequencyoffset (SFO) pre-compensation to their respective downlinktransmissions, as described further hereinbelow with reference to FIGS.5 and 6.

Once the appropriate signaling has been set up, AP1 and AP2 begin abeamforming (BF) setup operation 98. For this purpose, AP1 and AP2transmit an NDP 100 to STA1. These NDPs arrive at STA1 with a difference102 in propagation delay, which will be taken into account in computingthe beamforming parameters. (Subsequent synchronization of thetransmission of downlink packets by AP1 and AP2 can be facilitated bytransmission of a trigger packet from STA1, as described furtherhereinbelow with reference to FIG. 5.) In one embodiment, AP1 and AP2each transmit a separate NDP 100. STA1 then estimates BF information 104for each NDP separately and transmits a feedback frame 106 (FIG. 4B)with respect to each NDP. Feedback frame 106 may be of the form definedin sections 9.4.1.65 and 9.4.1.66 of the above-mentioned IEEE 802.11axstandard, which are also incorporated herein by reference.

If the process of separate transmission of NDP 100 by each of AP1 andAP2 is unduly time-consuming, AP1 and AP2 may alternatively send NDP 100together, in a single, synchronized transmission, as though NDP 100 weretransmitted from a single, “virtual AP.” This option may requireadjusting the number of antennas in use (Ntx) to the Nss capability ofSTA1. In this case, STA1 will estimate BF information 104 for thecombined, virtual AP and send feedback frame 106 to AP1.

After collecting feedback frames 106 from STA1, AP1 and AP2 compute BFparameters 108 for subsequent use in DMIMO transmission to STA1. Theseparameters dictate the relative amplitudes and phases of the signals tobe transmitted from each of the participating antennas 28. The BFparameters may be expressed in the form of a steering matrix to beapplied to the downlink signal vectors (as explained in greater detailin the above-mentioned U.S. Provisional Patent Application 62/783,144,particularly on pages 28-32). Additionally or alternatively, AP1 and AP2may apply the feedback from STA1 in computing precoding matrices to beapplied to the downlink signals. For various DMIMO transmissionimplementations to a single STA, the precoding matrix is the steeringmatrix and is computed so as to maximize the signal/noise ratio of thedownlink signals received by the STA. For various DL-MUMIMOimplementations, the precoding matrix is computed so as to minimizeinterference experienced by each STA due to the downlink signalstransmitted to other STAs.

AP1 and AP2 apply BF parameters 108 in transmitting downlink datapackets 112 to STA1. Each AP transmits the same signal vector, with thesame MAC parameters, in each packet, but with its own BF and precodingparameters applied to each antenna 28. In this manner, it is alsopossible for the APs together to transmit a larger number of datastreams (Nss) to STA1 than either AP1 or AP2 could transmit on its own,thus taking greater advantage of the available network capacity(particularly when STA1 has a large number of available antennas).Encoding of the signal vector may take place entirely within AP1 andAP2. In this case, controller 64 passes the data bits of the messagethat is to be transmitted to AP1 and AP2, along with the procedure andparameters to be applied identically by both of the APs in encoding thedata. Alternatively, to avoid duplicate computations, controller 64 maybroadcast the complex modulated symbols to AP1 and AP2, which then applythe respective BF parameters to complete the encoding and transmissionof packets 112.

STA1 receives and decodes packets 112 in order to extract transmitteddata 114. The choice of BF parameters and the use of CFO/SFOpre-compensation enable STA1 to decode the signals as though they hadoriginated from a single (virtual) AP. As explained earlier, there is noneed for the STA to exercise any decoding capabilities in extractingdata 114 beyond those that it would have applied to a MIMO transmissionfrom a single AP.

After successfully decoding the data, STA1 forms and transmits anacknowledgment (ACK) packet 116. STA1 typically transmits only a singleACK packet, which is received by both AP1 and AP2. Reception of ACKpacket 116 by the associated AP (i.e., AP1 in the present example) maybe considered sufficient to complete the data communicationsuccessfully. If AP1 does not receive ACK packet 116, it may invokeretransmission of downlink data packets 112 by both AP1 and AP2.Alternatively, both AP1 and AP2 may send acknowledgment reports 118 tocontroller 64 upon receiving ACK packet 116; and the controller maydetermine that the data communication was successful if either (or both)of AP1 and AP2 reports having received the ACK packet.

The embodiment described above uses explicit measurement of downlinkchannel properties and feedback from the client STA is computing the BFparameters. In an alternative embodiment, the BF parameters, includingthe steering matrix, may be computed on the basis of the uplink signals,assuming channel reciprocality. This sort of approach, referred to as“implicit beamforming,” is described in the above-mentioned U.S.Provisional Patent Application 62/783,144, particularly on pages 57-65.As explained in detail in this part of the provisional application, theimplicit beamforming involves a two-stage calibration process: Firstcalibration factors are computed among the antennas of eachparticipating AP individually, and then a common calibration factor iscomputed across the multiple participating APs.

FIGS. 5 and 6 are ladder diagrams showing messages exchanged in theprocess of synchronization between access points AP1 and AP2, inaccordance with embodiments of the invention. The embodiment of FIG. 5depicts AP-initiated DMIMO, whereas that of FIG. 6 depictsclient-initiated DMIMO. In other respects the two embodiments aresimilar, and the same reference numbers are used to refer to equivalentelements in both figures.

Ordinarily, when a STA receives signals from multiple antennas 28 forits associated AP, the carrier frequencies and sampling frequencies ofthe AP transceivers 58 can be assumed to be synchronized. (As notedearlier, the APs can be periodically resynchronized during operation ofsystem 20, and they may also synchronize on demand in response to uplinktrigger packets from client STAs.) The STA is thus able to compute asingle CFO value and a single SFO value, corresponding to the offsetsbetween the AP frequencies and those of the STA transceivers 46, andapply the CFO and SFO values to all the received downlink signals. Ingeneral, however, the clock, carrier frequency and sampling frequency oftransceivers 58 in AP2 are subject to drift and thus are not preciselysynchronized with those in AP1. To enable STA1 to apply the same CFO andSFO values to all of the downlink signals that it receives in DMIMOoperation, AP1 and AP2 estimate the discrepancy between their respectivecarrier and sampling frequencies and then apply CFO/SFO pre-compensationto correct the discrepancy. This pre-compensation procedure isillustrated in FIGS. 5 and 6.

In the AP-initiated case of FIG. 5, AP1 associates with STA1 and makes aselection 128 of AP2 to join in DMIMO transmission. AP1 then sends aDMIMO trigger 130 to STA1, for example in the form of a suitable PHYprotocol data unit (PPDU). STA1 sends a response 132 to this trigger,similarly in the form of a PPDU. Based on this response, both AP1 andAP2 measure the characteristics of the uplink channel and estimate theirCFO and SFO relative to STA1. Based on these estimates, AP1 and AP2apply appropriate CFO/SFO pre-compensation 134 to subsequent signalsthat they transmit. Optionally, STA1 may transmit a further NDP trigger136 to assist AP1 and AP2 in accurate channel acquisition andpre-compensation. As a further option (not shown in the figures), AP2may transmit and receive its own, separate signals to and from STA1 forpurposes of channel acquisition and pre-compensation.

Using the pre-compensated CFO and SFO, AP1 and AP2 together transmit acommon NDP 138 to STA1, which processes the received signals to estimateBF information 104 for the combined, virtual AP. STA1 sends a feedbackframe 140 to AP1, reporting the BF information. On this basis, AP1 andAP2 compute respective precoding matrices 142.

Controller 64 broadcasts data 144 to AP1 and AP2, for transmission toSTA1. The actual data transmission may optionally be initiated inresponse to a data trigger 146 from STA1, which can be useful inimproving synchronization. AP1 and AP2 then transmit downlink datasignals 148, while applying the appropriate pre-compensation andprecoding to enable efficient reception and processing on the clientside.

In client-initiated DMIMO, as shown in FIG. 6, the procedure is similarto that described above, but in this case STA1 is responsible forselection of a DMIMO group 160. After selecting the group of APs, STA1sends a DMIMO trigger 162 to AP1 and AP2, which use the received signalin CFO/SFO pre-compensation 134. The synchronization and beamformingprocedures then continue as described above.

FIG. 7 is a flow chart that schematically illustrates a method fordownlink multi-user MIMO (DL-MUMIMO) communications, in accordance withan embodiment of the invention. The signal flow for DL-MUMIMO is similarto that described above for DMIMO to a single STA, but is adapted toenable multiple APs to serve multiple STAs simultaneously.

Referring back to FIG. 1 by way of example, DL-MUMIMO includes a varietyof different use cases:

-   -   1. Both STA 30 and STA 32 associate with the BSS of AP 22 and        receive DMIMO service from the antennas of AP 24 together with        AP 22. AP 22 and AP 24 will compute and apply different,        respective beamforming parameters for downlink communications        with STA 30 and STA 32.    -   2. Both STA 30 and STA 34 associate with the BSS of AP 24. STA        30 receives DMIMO service from the antennas of AP 24 together        with AP 22, while STA 34 receives DMIMO service from the        antennas of AP 24 together with AP 26. Thus, the beamforming        parameters for downlink transmission to STA 30 will be computed        and applied by AP 24 and AP 22, while those for downlink        transmission to STA 34 will be computed and applied by AP 24 and        AP 26.    -   3. STA 30 associates with the BSS of AP 22, while STA 34        associates with the BSS of AP 26. Both STAs receive DMIMO        transmissions from their respective, associated AP together with        the antennas of AP 24.    -   4. STA 30 associates with the BSS of AP 22, while STA 32        associates with the BSS of AP 24. Both STA 30 and STA 32 receive        DMIMO service from AP 22 and AP 24. In addition to the        different, respective beamforming parameters for STA 30 and STA        32, AP 22 and AP 24 will also use different BSSIDs in the        downlink communications: The downlink packets transmitted from        AP 24 to STA 30 will use the BSSID of AP 22, while those        transmitted from AP 22 to STA 32 will use the BSSID of AP 24.

The method of FIG. 7 is initiated when client STAs, for example STA1 andSTA2, associate with respective APs (or possibly the same AP) in a WLAN,at an association step 180. Assuming an AP-triggered DMIMO model, asdefined above, controller 64 (FIG. 2) instructs the associated APs topoll some or all of their client STAs for STA reports, indicating theneighboring APs from which each client STA is able to receive downlinksignals. Based on this information, controller 64 defines a respectiveantenna group to serve each of STA1 and STA2, at a group selection step182. This selection involves at least one neighboring AP, in addition tothe associated AP, in accordance with whichever of the use-case modelslisted above is applicable.

The APs serving each STA collect beamforming information and computedownlink beamforming parameters for that STA, at a parameter computationstep 184. This step can be carried out individually for each STA insubstantially the same manner as was described above for the single-STAcase. The one possible exception is in terms of pre-compensation for CFOand SFO (as explained above in reference to FIGS. 5 and 6): It is notpractical for the APs in the WLAN to compute and apply a differentCFO/SFO pre-compensation value in downlink transmission to each STA.

Therefore, at step 184, one of APs (for example, AP1) is selected bycontroller 64 to serve as the master AP for purposes of CFO/SFO. AP1instructs each of the client STAs to transmit an NDP, as explainedabove. Both AP1 and the other APs in the WLAN receive the NDP and, onthis basis, calculate their respective frequency offsets with respect toeach client STA. AP1 computes baseline CFO and SFO values (for example,values that are averaged over the client STAs), and then broadcaststhese value to the other APs. These other APs perform CFO/SFOpre-compensation relative to the baseline values, and thus synchronizetheir carrier frequencies and sampling frequencies relative to AP1. Theclient STAs will still need to correct for CFO and SFO in the downlinkcommunications that they receive, but it will be the same correction forall APs in the network.

Following selection of the respective antenna group and computation ofthe respective beamforming parameters for each client STA, downlink datacommunications proceed in the fashion described above for single-clientDMIMO. Controller 64 distributes data for transmission to each of STA1and STA2 to the APs in the respective antenna group, at a datadistribution step 186. The participating APs encode and transmit thedata to each of STA1 and STA2 in mutual synchronization, at a datatransmission step 188. The participating APs apply joint multi-userprecoding to emulate downlink transmission from the entire group ofantennas, thus enable simultaneous transmissions to STA1 and STA2.

It is also possible to take advantage of the features of orthogonalfrequency divisional multiple access (OFDMA) that are provided by the802.11 standards in conjunction with DL-MUMIMO operation. In this case,the APs serving a given client station will use the same OFDMA resourceunit (RU) in transmitting to the client, and may also use the samepreamble. Different DL-MUMIMO antenna groups, serving different clients,may operate in the same RU or in different RUs.

Although the description above relates to downlink transmissions, DMIMOgroups may also be used advantageously in enhancing the quality ofuplink reception, by increasing the uplink diversity gain. For example,assuming AP1 to be the associated access point for STA1, and AP2 to havebeen joined with AP1 in a DMIMO group, AP2 will also receive uplinktransmissions from STA1. AP2 may decode these transmissions and computea soft metric with respect to the data values in the received uplinkpackets. AP2 reports these metrics to AP1 via backbone network 36. AP1combines these metrics with its own decoding results in order to decodethe data with greater accuracy.

It will be noted that the embodiments described above are cited by wayof example, and that the present invention is not limited to what hasbeen particularly shown and described hereinabove. Rather, the scope ofthe present invention includes both combinations and subcombinations ofthe various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. A method for communication in a WLAN,comprising: associating a client station (STA) with a basic service set(BSS) of a first access point (AP), having first antennas, in a wirelesslocal area network (WLAN); synchronizing a second AP, having secondantennas, in the WLAN with the first AP; computing distributedbeamforming parameters over a group of the first antennas and the secondantennas; distributing data for transmission to the STA to both thefirst AP and the second AP; and transmitting the distributed data to theSTA from the first AP via the first antennas in the group and the secondAP via the second antennas in the group in synchronization in accordancewith the distributed beamforming parameters.
 2. The method according toclaim 1, wherein synchronizing the second AP comprises exchangingsynchronization signals between the first AP and the second AP over abackbone network.
 3. The method according to claim 2, wherein exchangingthe synchronization signals comprises synchronizing respective clocksand frequency offsets of the second AP with the first AP.
 4. The methodaccording to claim 1, wherein associating the STA comprises providingfrom the first AP to the STA a BSS identifier (BSSID) of the first AP,and wherein transmitting the distributed data comprises transmittingsignals from both the first AP and the second AP using the BSSID of thefirst AP.
 5. The method according to claim 4, wherein transmitting thesignals comprises conveying encoding parameters to the second AP thatare identical to the encoding parameters applied by the first AP, andencoding the distributed data for transmission by both the first AP andthe second AP using the identical encoding parameters.
 6. The methodaccording to claim 1, wherein computing the distributed beamformingparameters comprises receiving from the STA, at the first AP, a requestto establish the group of the antennas together with the second AP, andsetting up the group in response to the request.
 7. The method accordingto claim 1, wherein computing the distributed beamforming parameterscomprises collecting information at the first AP with respect toneighboring APs in the WLAN, and selecting the second AP responsively tothe collected information.
 8. The method according to claim 1, whereincomputing the distributed beamforming parameters comprises transmittingnull data packets (NDPs) from both the first AP and the second AP to theSTA, receiving channel feedback from the STA in response to the NDPs,and computing, responsively to the channel feedback, a steering matrixto be applied to downlink signal vectors.
 9. The method according toclaim 1, wherein computing the distributed beamforming parameterscomprises receiving uplink signals from the STA at the first AP and thesecond AP, and computing, by implicit beamforming based on the uplinksignals received at both the first AP and the second AP, a steeringmatrix to be applied to downlink signal vectors.
 10. The methodaccording to claim 9, wherein computing the steering matrix by implicitbeamforming comprises computing calibration factors among the antennasof each of the first and second APs individually, and then computing acommon calibration factor across both of the first and second APs. 11.The method according to claim 1, wherein transmitting the distributeddata comprises encoding the data for transmission at both the first APand the second AP identically in accordance with a specified procedureand encoding parameters.
 12. The method according to claim 1, whereindistributing the data for transmission comprises broadcasting complexmodulated symbols, which encode the data, to be transmitted by the firstAP and the second AP.
 13. The method according to claim 1, whereintransmitting the distributed data comprises applying, by at least one ofthe APs, a pre-compensation for a frequency offset in signals that aretransmitted from the APs to the STA.
 14. The method according to claim1, wherein computing the distributed beamforming parameters comprisesdetermining amplitude and phase values to be applied in transmitting thedata via each of the first and second antennas.
 15. The method accordingto claim 1, wherein computing the distributed beamforming parameterscomprises computing precoding matrices to be applied to the data fortransmission.
 16. The method according to claim 1, wherein associatingthe STA comprises associating at least a first STA and a second STA withthe BSS of the first AP, and wherein computing the distributedbeamforming parameters comprises computing different, respective firstand second beamforming parameters for downlink communications with thefirst STA and the second STA.
 17. The method according to claim 16,further comprising synchronizing a third AP, having third antennas, inthe WLAN with the first AP, wherein computing the different, respectivefirst and second beamforming parameters comprises computing the firstbeamforming parameters over a first group of the first antennas and thesecond antennas, and computing the second beamforming parameters over asecond group of the first antennas and the third antennas, and whereintransmitting the distributed data comprises transmitting first signalsto the first STA from the first and second APs in accordance with thefirst beamforming parameters and transmitting second signals to thesecond STA from the first and third APs in accordance with the secondbeamforming parameters.
 18. A wireless local area network (WLAN) systemfor communication, comprising: a first access point (AP), having firstantennas and a basic service set (BSS), which is configured to associatewith a client station (STA) in a wireless local area network (WLAN); asecond AP in the WLAN, having second antennas, which is configured tosynchronize with the first AP; and at least one processor, which isconfigured to compute distributed beamforming parameters over a group ofthe first antennas and the second antennas, and to distribute data fortransmission to the STA to both the first AP and the second AP, whereinthe first AP and the second AP transmit the distributed data to the STAvia the first antennas in the group and the second antennas in thegroup, respectively, in synchronization in accordance with thedistributed beamforming parameters.
 19. The system according to claim18, wherein the first AP and the second AP are connected over a backbonenetwork, and wherein the second AP is synchronized with the first AP byexchanging synchronization signals between the first AP and the secondAP over the backbone network prior to transmitting the distributed datato the STA.
 20. The system according to claim 18, wherein the first APis configured to provide to the STA a BSS identifier (BSSID) of thefirst AP, and wherein the distributed data are transmitted from both thefirst AP and the second AP to the STA using the BSSID of the first AP.